1. Field of the Invention
This invention relates to optoelectronic modules. The optoelectronic module mean an LD module, a PD module, an LD/PD module, a set of them.
This application claims the priority of Japanese Patent Application No. 2000-117896 filed on Apr. 19, 2000 which is incorporated herein by reference.
2. Description of Related Art
Current optical communication systems utilize three-dimensionally shaped metal-packaged optoelectronic modules having a cylindrical metal package, a PD chip, an LD chip, a lens, a ferrule and a fiber which align along the central axial line perpendicular to the metallic stem. The current module requires a lens for converging light since the light propagates for a long distance which diverges the light beam in the module. The currently used optoelectronic modules excel in the sealing property, the noise-resistance and the reliability. The current LD, PD or LD/PD modules, however, have drawbacks of complicated alignment, large size and high-cost. The bulky prior modules occupy a wide area in a print circuit board.
A further prevalence of the optical communication networks requires smaller sizes and lower cost for LD, PD or LD/PD modules. Recently contrived PLC (Planar Lightguide Circuit) is promising technology for accelerating the size-reduction and cost-curtailment of optoelectronic modules. In the PLC devices, the direction of the light propagation is in parallel with the surface of the substrate. The PLC has a small two-dimensional structure instead of the prior three-dimensional cylindrical structure. The PLC modules have strong points of the omission of alignment and the omission of the lens. Eliminations of the alignment and the lens are advantages of the PLC.
An example of a PLC type PD module is depicted in FIG. 1 (plan view) and FIG. 2 (vertical section. The PLC module is built upon a flat Si bench instead of the tall, cylindrical metallic package. A flat silicon bench 1 has a longitudinal bigger V-groove 2 and a smaller V-groove 3 along the center line. A ferrule 4 is inserted into the bigger V-groove 2 and the fiber 5 is inserted into the smaller V-groove 3. An adhesive fixes the ferrule 4 and the fiber 5 to the grooves. An end of the Si bench is cut to make a lower stage 6 for revealing the front end of the V-groove 3. The lower stage 6 is provided with an optoelectronic device which is a waveguide type PD 7 in the example. The waveguide type PD 7 which has a horizontally spreading sensing layer 8 and a waveguide layer upon the sensing layer allows the light to go into the PD via a front end. The waveguide type simplifies the structure of the PD module.
The example in FIGS. 1 and 2 shows the waveguide type PD upon the substrate because of the simplicity of the structure. The PLC structure allows also a bottom incidence type PD or a top incidence type PD to build a PD module on a flat substrate. The PDs take slightly different relations to the optical fiber axis in the case of the bottom incidence type PD or the top incidence type PD. Someone proposed transmitting modules disposing light sources LED or LD) at extensions of optical fibers upon substrates. A PD module, an LD/PD module and an LD module take similar flat PLC structures. Sometimes the optical fibers are replaced by light waveguides formed upon the substrates (Si-benches) and coupled to outer optical fibers at the end of the substrate.
In any cases, the PLC structure has a substrate, at least an optical fiber or a waveguide on the substrate and at least one optoelectronic device. (PD, LD, LED or APD) disposed at an extension of the fiber or the waveguide upon the substrate. Without a lens, the optical fiber or the waveguide is directly butted to the optoelectronic device chips (PD, LD, LED or APD), which reduces the number of parts and decreases the size of the module.
The prior art of FIGS. 1 and 2 determines the position of the fiber by etching a V-groove upon the substrate and exactly determines the position of the PD by putting the PD at characteristic marks printed on the substrate. Without active alignment, the exact positions of the fibers and the optoelectronic devices are determined by the V-grooves and the characteristic marks designated upon the substrate. The facile positioning in the PLC is called xe2x80x9cpassive alignmentxe2x80x9d in contrast with the xe2x80x9cactive alignmentxe2x80x9d in the prior art current three dimensional metal packaged modules.
The PLC allows the passive alignment to the optical communication modules, which alleviates the cost of mounting. The PLC further will give low-cost modules by reducing the material cost and the parts cost in addition to the mounting cost. The reduction of the size will facilitate the mounting of the modules on a print circuit board. The PLC is a promising type of optical communication modules. However, the PLC must conquer several difficult problems for being put in practice. The current problems are here described with regard to the PD module shown in FIG. 1 and FIG. 2. Similar problems accompany the LD modules or the LD/PD modules.
The reflection at the end of the fiber is one of the problems of the receiving (PD) module of FIGS. 1 and 2. The end of the fiber is rectangular to the fiber axis. The difference of the refractive indexes between the fiber and the outer space reflects the light at the end of the fiber. The reflected light goes back in the fiber 5 and returns to the laser at the signal-sending port. The returning light induces instability on the oscillation of the laser by perturbing the stimulated oscillation. The quantity of the returning light depends upon the refractive index difference between the fiber and the outer space. The front end of the PD 7 is coated with an antireflection film which annihilates the reflection at the PD front end. What matters is the reflection at the fiber end.
The light reflected at the fiber end is called xe2x80x9creflection lightxe2x80x9d. The rate of the reflection light to the original incidence light is named xe2x80x9cOptical Return Loss (ORL)xe2x80x9d. The ORL is defined by
ORL=10 log(Pr/Pin), (dB)xe2x80x83xe2x80x83(1)
where Pin is the light power travelling in the fiber to the end and Pr is the light power reflected at the end. The light power which goes out of the fiber is (Pinxe2x88x92Pr). Since Pr less than Pin, the ORL is always negative. An ideal ORL is minus infinitive. In actual cases, the ORLs take definite minus values.
Even in vertical-cut end fibers, the ORL varies as a function of the refractive index difference between the fiber and the outer medium. In the case of silica (SiO2) fibers (n1=1.46), large reflection occurs when the outer medium is air (n0=1.0). The vertical reflection ratio Ref at the interface of the media of n0 and n1 is given by
Ref={(n1xe2x88x92n0)/(n1+n0)}2+0.035xe2x80x83xe2x80x83. (2)
The air/silica interface brings about ORL=xe2x88x9214.6 dB which is too big for practical use.
The allowable upper limit of ORL depends upon the systems (LD, PD or LD/PD modules). PD modules require an ORL less than xe2x88x9227 dB as a whole. Assuming that the manufacturing margin is xe2x88x923 dB, the ORL should be less than xe2x88x9230 dB at the interface. Such a small ORL (e.g., ORLxe2x89xa6xe2x88x9230 dB) is required in a wide temperature range between xe2x88x9240xc2x0 C. and +85xc2x0 C. for the PD modules. The less than xe2x88x9230 dB ORL in the wide temperature range is a quite rigorous condition.
The prior PD module of FIGS. 1 and 2 cannot suffice the severe requirement. The use of the PD module is narrowly restricted. Some contrivance is requested for enhancing the utility of the FIG. 1 module.
Eq. (2) notifies us that a reduction of the refractive index difference should be effective for reducing the reflection loss at the fiber end. FIG. 3 and FIG. 4 show an improvement of potting a transparent resin having a refractive index n2 similar to the fiber into the gap between the fiber 5 and the PD chip 7. The transparent potting resin 9 alleviates the reflection at the interface between the fiber and the resin. The reflection rate is given by replacing n0 (air) with n2 of the transparent resin 9 in Eq. (2).
The prevalent potting resins are silicone-type resins and acrylate-type resins which are transparent to the 1.3 xcexcm light and the 1.55 xcexcm light which are typical wavelengths in the optical communication systems. Some of the silicone-type and the acrylate-type resins have a refractive index similar to the silica optical fiber (n1=1.46). For example, a thermosetting silicone type resin has a refractive index n=1.4 at room temperature which is similar to the silica fiber (n1=1.46). An U (ultraviolet rays)-setting acrylate type resin has a refractive index n=1.5 at room temperature which is also close to the fiber. Only the two resins among the known transparent resins have close refractive indexes to the SiO2 fibers.
When the modules are kept at room temperature, both the transparent UV-setting acrylate resin and the thermosetting silicone resin satisfy the requirement of the ORL less than xe2x88x9230 dB. The refractive indexes of the resins vary as a function of the surrounding temperature. The temperature at which the modules should normally operate has a wide range from xe2x88x9240xc2x0 C. to +85xc2x0 C. A change of the temperature would increase the ORL through a variation of the resin refractive index. The wide range (from xe2x88x9240xc2x0 C. to +85xc2x0 C.) of the allowable temperature increases the difficulty of finding the resin satisfying the condition of the ORL less than xe2x88x9230 dB. Nobody knows such a convenient resin yet.
FIG. 5 shows the ORL (dB) of the modules having the fiber and the PD chips covered with the current-known potting resins of a UV-setting acrylate resin (⋄) and a thermosetting silicone resin (◯) as a function of the temperature.
The thermosetting silicone resin (◯) takes the minimum ORL of xe2x88x9239 dB at the lowest temperature (xe2x88x9240xc2x0 C.) in the range (from xe2x88x9240xc2x0 C. to +85xc2x0 C.). An increase of the temperature raises the ORL for the thermosetting silicone resin. The ORL raises to xe2x88x9234 dB at 25xc2x0 C. The highest temperature +85xc2x0 C. gives the resin a high ORL of xe2x88x9230 dB. The temperature dependence (◯) of the ORL is caused by the refractive index of the thermosetting silicone resin which is the nearest (1.40) at xe2x88x9240xc2x0 C. to the fiber refractive index (1.46) but decreases with a rise of the temperature. The thermosetting silicone resin (◯) cannot satisfy the inequality ORLxe2x89xa6xe2x88x9230 dB in the vicinity of 85xc2x0 C., when the manufacturing margin is taken into account.
The UV-setting acrylate resin (⋄) takes the minimum ORL of xe2x88x9237 dB at the highest temperature +85xc2x0 C. on the contrary. The ORL rises with a decrease of the temperature. The ORL is xe2x88x9234 dB at room temperature (25xc2x0 C.). The ORL is enhanced to xe2x88x9230 dB at the lowest temperature xe2x88x9240xc2x0 C. The temperature variation derives from the refractive index which becomes the nearest to the fiber (1.46) at 85xc2x0 C. but rises with the decrease of the temperature. The UV-setting acrylate resin (⋄) cannot satisfy the requirement ORL less than xe2x88x9230 dB in the lower limit of xe2x88x9240xc2x0 C. with the manufacturing margin. Both the UV-setting acrylate resin (⋄) and the thermosetting silicone resin (◯) are not satisfactory yet as a potting resin for the rigorous criterion of ORLxe2x89xa6xe2x88x9230 dB in the range from 40xc2x0 C. to +85xc2x0 C.
The temperature dependent variation of the ORL in resin-coated PD modules derives from the temperature dependent change of the refractive index of the coating resins. Some of the thermosetting silicone resins have refractive indices which vary from 1.48 to 1.37 in the temperature range between xe2x88x9240xc2x0 C. and 85xc2x0 C. Some of the UV-setting acrylate type resins have refractive indices which vary from 1.56 to 1.49 in the same temperature range. The temperature dependent variations of refractive indices prohibit nearly all the current-known resins from satisfying the ORL condition of less than xe2x88x9230 dB in the wide temperature range from xe2x88x9240xc2x0 C. to +85xc2x0 C. An ideal resin should have a refractive index at a certain temperature nearly equal to the refractive index of the fiber and should have a sufficiently small temperature coefficient (xcex4n/xcex4T; T is the temperature). The resin should have also some physical rigidity when it is hardened(set). Such an ideal resin does not exist at present.
The Inventors have found a candidate resin which can satisfy the requirement of the small ORL in the determined temperature range but is not endowed with rigidity for protecting chips or fibers. The candidate resin the Inventors discovered is one of the UV-setting silicone type resins. The refractive index of the candidate is very close to the refractive index of the silica (SiO2) optical fiber and the temperature variation of the refractive index is also similar to that of the silica fiber. The advantage of the candidate resin is the double similarities to the SiO2 fibers with regard to the refractive index (n) and the temperature dependence (xcex4n/xcex4T) of the refractive index. In addition, the refractive index of the candidate resin crosses the refractive index of the fiber at a certain temperature within the determined temperature range. The crossing is a favorite property of the candidate. The refractive index crossing reduces the ORL to a negative infinitive just at the crossing temperature and maintains the ORL in quite small values in the vicinity of the crossing temperature. Fortunately, the strong fall of the ORL at the crossing temperature reduces the ORL sufficiently in all the temperature range between xe2x88x9240xc2x0 C. and +85xc2x0 C. xcex94 dots show the ORL measured for the candidate resin in FIG. 5 in test modules having a PD, a fiber and a candidate resin layer covering the PD and the fiber.
The candidate ORL falls to xe2x88x9266 dB near 0xc2x0 C. The sink means that the refractive index crossing occurs in a vicinity of 0xc2x0 C. between the fiber and the candidate resin. An exact coincidence would bring about a negative infinitive for the ORL. Discrete measuring points are restricted to xe2x88x9240xc2x0 C., 0xc2x0 C., +25xc2x0 C., +75xc2x0 C. and +85xc2x0 C. in FIG. 5. The crossing temperature should lie slightly below 0xc2x0 C. Both a rise or a fall of temperature from the minimum point (crossing point) raises the ORL through an increase of the refractive index differences. The fact that the refractive index has a crossing point (n1=n2) within the required temperature range (xe2x88x9240xc2x0 C.xcx9c+85xc2x0 C.) is a favorite property of the candidate. The minimum strongly pulls down the ORL curve far smaller than xe2x88x9230 dB overall. Indeed, the maximum at +85xc2x0 C. gives a xe2x88x9239 dB ORL which satisfies the condition.
The description may not be understood easily. Silicone type resins have several different species. The silicone type is further divided by the way of hardening. The resins which are hardened by ultraviolet rays are called UV-setting silicone type resins. The resins which are hardened by heat are called thermosetting silicone type resins. Both groups also have some species. The refractive indexes of the resins are varied by changing the ratios of the components.
The refractive index is an important factor of choosing the pertinent resin for covering the devices in the modules. But the refractive index is not a unique criterion of determining the resin. Physical property is another important factor. Some rigidity is required for the coating resin. For example, a kind of the UV-setting silicone resins has a refractive index quite close to the refractive index of the optical fiber. The UV-setting silicone resin is plagued by a physical weak point. The UV-setting silicone resin is gelled instead of being hardened. The UV-set silicone resin has fluidity and is still unstable in shape after the UV-setting. The shape of the gelled silicone resin is changed by external pressure or external forces. If the unstable UV-setting resin is used for covering the PD, the fiber and the inner space in FIG. 3 or FIG. 4, the resin can not maintain the solid shape against the epoxy resin. When the silicone resin with the substrate is covered with the epoxy molding resin, the fluid UV-setting resin will be swept away from the inner space between the PD and the fiber by the pressure of the epoxy resin. Although the UV-setting silicone resin is the most favorable one from the refractive index, the resin cannot be utilized as the transparent resin for covering the PD, the fiber and the interval of them due to the fluidity after setting.
The optoelectronic module of the present invention has a triple resin structure comprising an inner resin, a medium resin and an outer resin. Among the three resins, the inner resin is the most important. The inner resin should have a refractive index nearly equal to the optical fiber or the optical waveguide. The inner resin is gelled by some means but has some fluidity after setting. The inner resin is, for example, a gelled UV-setting silicone resin or a fluid silicone matching oil having a refractive index nearly equal to the fiber/waveguide. The inner resin which cannot maintain the shape by itself should be stored in a cavity and enclosed further by the medium resin. The cavity and the medium resin determine the shape of the inner resin by enclosing the inner resin. The medium resin should have some rigidity for protecting the fluid inner resin from the outer resin. The outer resin is a hard resin for maintaining the whole shape of the module.
The triplet resin structure is made on a substrate by boring a cavity at the joining region of the optoelectronic device and a fiber/waveguide on the substrate, supplying an inner resin (e.g., a gelled UV-setting silicone resin or a fluid silicone matching oil) having a refractive index nearly equal to the fiber/waveguide into the cavity, potting a medium resin on the inner resin, the PD, the fiber and the substrate for protecting the inner resin. The shape of the inner resin is determined by the cavity and the medium resin. The resin-keeping pond (cavity) can be utilized as an optical path between the fiber/waveguide and the optoelectronic chip by contriving the light path. Furthermore, the V-grooves for sustaining optical fibers upon the substrate can be also utilized as a part of the cavity for keeping the inner resin. In the case, the ready-made grooves can be employed for storing the inner resin.
In the case of the substrate having no V-grooves for the fibers, a cavity (pond) should be made on the substrate for maintaining the fluid inner resin. In this case, the resin pond prohibits the fluid transparent (gelled) resin from flowing away from the interval region of the PD and the fiber/waveguide.
The gelled inner resin cannot maintain the shape due to the fluidity. Then, the inner resin with the optoelectronic chips (PD, LD, LED ) should be enclosed by a harder medium resin. The medium resin should be endowed with medium elasticity not to apply strong stress upon the optoelectronic chips or other optical parts. High resistivity is another property required for the medium resin, because the medium resin comes in contact with electrodes and wiring patterns. Candidates for the medium resin are, e.g., thermosetting silicone resin and UV-setting acrylate resin. The elasticity prevents the medium resin from maintaining the inherent shape for a long time. The substrate is fitted to a member, for example, a lead frame, for accomplishing electrical connection between the optoelectronic devices and external circuits.
Then, the medium resin should be further enclosed by the hard outer resin, for example, an epoxy resin for keeping the determined shape everlastingly. The outer resin should be endowed with high rigidity, waterproofness (sealing) and high resistivity (insulator). A suitable outer resin is an epoxy resin which has been used for the material of the plastic packages of electronic devices. The module is packaged by storing the substrate and the lead frame covered with the medium resin into a case, molding the substrate with the lead frame by the outer resin in the case and capping the case. Otherwise, the module is directly plastic-molded by inserting the substrate with the medium resin into a metallic mold, injecting fluid resin and hardening the resin by heating or cooling in the mold.
The present invention has two features of the transparent fluid, gelled resin stored in a cavity upon the substrate and the triplet resin structure. The gelled, fluid transparent resin reduces the reflection at the end of the fiber/waveguide by the refractive index which is nearly equal to the fiber in the wide temperature range. The transparent resin is unstable by itself but the resin is doubly protected by the medium resin and the outer resin. The double enclosure stabilizes the inner resin. The inner resin allows the modules to maintain the ORL enough below the desired value (e.g., ORLxe2x89xa6xe2x88x9230 dB) in the wide temperature range. The present invention can provide inexpensive small-sized optoelectronic modules (PD modules, LD modules and LD/PD modules) maintaining low ORL in the whole required temperature range.
Why has nobody hit on an idea implying the present invention? All the known resins which maintain a permanent shape after setting cannot satisfy the requirement of ORLxe2x89xa6xe2x88x9230 dB for PD modules in the wide temperature range (xe2x88x9240xc2x0 C.xcx9c+85xc2x0 C.). Few people have noticed that there are some fluids which can satisfy the rigorous requirement but cannot be hardened completely. Even if anybody had noticed such a fluid, they would have thought that the fluid or the gelled resin could not fill the optical path between the fiber/waveguide and the PD/LD.
The Inventors denied the common sense and tried to make use of the fluidity of the resin or the oil having the suitable refractive index for reducing the ORL. A fluid can be maintained by storing the fluid in a vessel, a cup or a container. A PLC substrate has no room for placing an extra vessel. But a substantial vessel can be given to the substrate by boring a cavity. The cavity would keep the fluid on the substrate. If the fluid-storing cavity is made along the light path between the fiber/waveguide and the PD/LD, the cavity would be effective to reduce the reflection. The flow of the fluid is permanently killed by covering (capping) the fluid with a resin which can be hardened by some means. The capping resin. should be electrically insulating and be stable electrically and thermally. The capping (medium) resin should be elastic even after being set. The elasticity protects the PD, the LD or the AMP from external stress. A thermosetting silicone resin or a UV-setting acrylate resin is suitable for the capping resin. The capping resin cannot be the outermost material due to the elasticity.
The shell should be formed with a harder resin. An epoxy resin is suitable for the outermost resin. The epoxy resin is opaque and rigid after being hardened. The rigidity maintains its own shape permanently. The excess rigidity would damage or degrade inner devices because of strong inner stress caused by the temperature variation. But the present invention encloses the PD, the LD or the ICs by the elastic medium resin which alleviates the stress from the rigid outer resin. The medium resin protects the devices from the external stress or the external force by deforming itself The triple resin layers give such advantages to the optoelectronic modules.